This invention relates to lower tie plates for nuclear fuel bundles. In order to understand this invention, the construction of a nuclear fuel bundle will be first summarized. Thereafter, the support of fuel bundles on the fuel support casting interior of a nuclear reactor will be set forth. Finally, the difference between the so-called "D" and "C" lattices will be discussed--emphasizing the "D" lattice mounting as non-symmetrical and the "C" lattice mounting as symmetrical. This summary will set the background required to understand the subject two part tie plate construction which can accommodate both "D" and "C" lattices.
For purposes of this invention, the construction of a nuclear fuel bundle can be easily summarized. It includes a lower tie plate supporting an upstanding, vertical, side-by-side array of sealed fuel rods at a rod supporting grid. This lower tie plate functions to admit moderating water coolant into the fuel bundle.
The vertical fuel rods are bound in side-by-side relationship by so-called fuel rod spacers with the bundle terminating in an upper tie plate. This upper tie plate allows the water moderator and generated steam to escape from the fuel bundle. Additionally, using certain of the fuel rods also referred to as "tie rods", the upper and lower tie plates are held together making the fuel bundle a unitary assembly which can be moved to and from locations in the reactor.
Finally, the fuel bundle is surrounded by a fuel bundle channel which extends from the vicinity of the lower tie plate to the vicinity of the upper tie plate. This channel assures that the two phase flow of water and steam through the fuel bundle is separated from the so-called surrounding "core bypass" region which is normally filled with water. This same core bypass region is the region in which the so-called "control rods" pass to absorb neutrons and control the nuclear reaction within the fuel rods.
Having summarized fuel bundle construction, a simplified explanation of the support of the fuel bundles may now be offered.
Fuel bundles interior of a nuclear reactor are supported in groups of four on fuel support castings. Such support occurs at each fuel bundle at the lower tie plate. The fuel support casting distributes the moderating water coolant to the lower tie plates and is provided with a central aperture through which the control rod moves into and out of the interstitial space between the fuel bundle. When the control rod is out of the interstitial space between the fuel bundles, water occupies the control rod interval and assists in promoting the nuclear reaction within the supported fuel bundles. When the control rod is in the interstitial space between the fuel bundles, water is displaced in the control rod interval and the control rod absorbs neutrons.
Having given the simplified explanation of the support of the fuel bundles, the difference between the "D" and "C" lattices can now be set forth.
In certain boiling water nuclear reactors, the spacial interval between adjacent fuel bundles was non-uniform. Taking the case of adjacent fuel bundles, if the space between the fuel bundles was one into which a control rod passed, a "large" separation between the fuel bundles was provided. If the space between the fuel bundles was one into which a control rod did not penetrate, a "small" separation between the fuel rods was provided. These boil water nuclear reactors were referred to as "D" lattice reactors.
It will be understood that the terms large and small as used herein are relative. In actual fact, the change in spatial interval was quite small--being in the order of 0.040 of an inch parallel to the sides of the fuel bundles. This change in spacing was provided by an eccentricity built into the lower tie plate. That eccentricity can be easily understood.
Lower tie plates have a nozzle section which directly rests upon the core support casting. In a D lattice lower tie plate, this nozzled section was offset with respect to the rod supporting grid section of the tie plate. This offset occurred away from the centerline of the fuel bundle supporting casting. The spatial interval between fuel bundles where the control rod penetrated was larger than the smaller spatial interval between fuel bundles where the control rod did not penetrate.
In later boiling water nuclear reactors, this eccentricity of mounting the fuel bundles was abandoned. In a so-called "C" latticed lower tie plate, this nozzled section was concentric with respect to the rod supporting grid section of the tie plate. No offset occurred away from the centerline of the core support casting. The spatial interval between fuel bundles where the control rod penetrated was the same as the spatial interval between fuel bundles where the control rod did not penetrate.